Place the onion, chili beans, black beans, corn, tomato sauce, beer and diced tomatoes in slow cooker.
Add taco seasoning and stir to blend
Lay chick breasts on top of the mixture, pressing down slightly until just covered.
Cover and cook on "low" for 5 hours
Remove chicken breast, shred and place back into the pot
Cook for 2 more hours
Service topped with cheese, sour cream, and tortilla chips